We propose to prepare a series of hybrid cell lines that produce monoclonal human antibodies against the major envelope proteins of cytomegalovirus (CMV). These hybrids wil be prepared by the fusion of either mouse or human myeloma cells with the lymphocytes of patients that are undergoing an active immune response to CMV. Hybrid cells producing anti-CMV antibodies will be cloned and propogated in culture as a source of homogeneous immunoglobulin. After the appropriate demonstration of specificity and biological activity against CMV, these monoclonal antibodies will be used for the passive immunization of a selected group of patients (notably those from our bone marrow transplantation study) who have an unusually high frequency of CMV-induced pneumonia. In fact, CMV activation occurs in 47% of these patients within a 100 day period after whole body irradiation, and approximately half of these individuals progress to a fatal pneumonia. Of particular interest is the finding that individuals who produce high titers of anti-CMV antibody after transplantation are less likely to succumb to fatal CMV-associated pneumonia. Thus, this viral system is uniquely suited for studies on the effect of passive immunization with high doses of antibody.